This PEP adds a new attribute fold to instances of the
datetime.time and datetime.datetime classes that can be used
to differentiate between two moments in time for which local times are
the same. The allowed values for the fold attribute will be 0 and 1
with 0 corresponding to the earlier and 1 to the later of the two
possible readings of an ambiguous local time.

In most world locations, there have been and will be times when
local clocks are moved back. [1] In those times, intervals are
introduced in which local clocks show the same time twice in the same
day. In these situations, the information displayed on a local clock
(or stored in a Python datetime instance) is insufficient to identify
a particular moment in time. The proposed solution is to add an
attribute to the datetime instances taking values of 0 and 1 that
will enumerate the two ambiguous times.

People who live in locations observing the Daylight Saving
Time (DST) move their clocks back (usually one hour) every Fall.

It is less common, but occasionally clocks can be moved back for
other reasons. For example, Ukraine skipped the spring-forward
transition in March 1990 and instead, moved their clocks back on
July 1, 1990, switching from Moscow Time to Eastern European Time.
In that case, standard (winter) time was in effect before and after
the transition.

Both DST and standard time changes may result in time shifts other
than an hour.

When clocks are moved back, we say that a fold[2] is created in time.
When the clocks are moved forward, a gap is created. A local time
that falls in the fold is called ambiguous. A local time that falls
in the gap is called missing.

We propose adding an attribute called fold to instances of the
datetime.time and datetime.datetime classes. This attribute
should have the value 0 for all instances except those that represent
the second (chronologically) moment in time in an ambiguous case. For
those instances, the value will be 1. [3]

An instance that has fold=1 in a non-ambiguous case is
said to represent an invalid time (or is invalid for short), but
users are not prevented from creating invalid instances by passing
fold=1 to a constructor or to a replace() method. This
is similar to the current situation with the instances that fall in
the spring-forward gap. Such instances don't represent any valid
time, but neither the constructors nor the replace() methods
check whether the instances that they produce are valid. Moreover,
this PEP specifies how various functions should behave when given an
invalid instance.

The __new__ methods of the datetime.time and
datetime.datetime classes will get a new keyword-only argument
called fold with the default value 0. The value of the
fold argument will be used to initialize the value of the
fold attribute in the returned instance.

The replace() methods of the datetime.time and
datetime.datetime classes will get a new keyword-only argument
called fold. It will behave similarly to the other replace()
arguments: if the fold argument is specified and given a value 0
or 1, the new instance returned by replace() will have its
fold attribute set to that value. In CPython, any non-integer
value of fold will raise a TypeError, but other
implementations may allow the value None to behave the same as
when fold is not given. [4] (This is
a nod to the existing difference in treatment of None arguments
in other positions of this method across Python implementations;
it is not intended to leave the door open for future alternative
interpretation of fold=None.) If the fold argument is not
specified, the original value of the fold attribute is copied to
the result.

The datetime.now() method called without arguments will set
fold=1 when returning the second of the two ambiguous times in a
system local time fold. When called with a tzinfo argument, the
value of the fold will be determined by the tzinfo.fromutc()
implementation. When an instance of the datetime.timezone class
(the stdlib's fixed-offset tzinfo subclass,
e.g.datetime.timezone.utc) is passed as tzinfo, the
returned datetime instance will always have fold=0.
The datetime.utcnow() method is unaffected.

A new feature is proposed to facilitate conversion from naive datetime
instances to aware.

The astimezone() method will now work for naive self. The
system local timezone will be assumed in this case and the fold
flag will be used to determine which local timezone is in effect
in the ambiguous case.

An implication is that datetime.now(tz) is fully equivalent to
datetime.now().astimezone(tz) (assuming tz is an instance of a
post-PEP tzinfo implementation, i.e. one that correctly handles
and sets fold).

The timestamp() method of datetime.datetime will return different
values for datetime.datetime instances that differ only by the value
of their fold attribute if and only if these instances represent an
ambiguous or a missing time.

When a datetime.datetime instance dt represents an ambiguous
time, there are two values s0 and s1 such that:

datetime.fromtimestamp(s0) == datetime.fromtimestamp(s1) == dt

(This is because == disregards the value of fold -- see below.)

In this case, dt.timestamp() will return the smaller of s0
and s1 values if dt.fold == 0 and the larger otherwise.

When a datetime.datetime instance dt represents a missing
time, there is no value s for which:

datetime.fromtimestamp(s) == dt

but we can form two "nice to know" values of s that differ
by the size of the gap in seconds. One is the value of s
that would correspond to dt in a timezone where the UTC offset
is always the same as the offset right before the gap and the
other is the similar value but in a timezone the UTC offset
is always the same as the offset right after the gap.

The value returned by dt.timestamp() given a missing
dt will be the greater of the two "nice to know" values
if dt.fold == 0 and the smaller otherwise.
(This is not a typo -- it's intentionally backwards from the rule for
ambiguous times.)

Users of pre-PEP implementations of tzinfo will not see any
changes in the behavior of their aware datetime instances. Two such
instances that differ only by the value of the fold attribute will
not be distinguishable by any means other than an explicit access to
the fold value. (This is because these pre-PEP implementations
are not using the fold attribute.)

On the other hand, if an object's tzinfo is set to a fold-aware
implementation, then in a fold or gap the value of fold will
affect the result of several methods:
utcoffset(), dst(), tzname(), astimezone(),
strftime() (if the "%Z" or "%z" directive is used in the format
specification), isoformat(), and timetuple().

The value of the fold attribute will only be saved in pickles created
with protocol version 4 (introduced in Python 3.4) or greater.

Pickle sizes for the datetime.datetime and datetime.time
objects will not change. The fold value will be encoded in the
first bit of the 3rd byte of the datetime.datetime
pickle payload; and in the first bit of the 1st byte of the
datetime.time payload. In the current implementation[8]
these bytes are used to store the month (1-12) and hour (0-23) values
and the first bit is always 0. We picked these bytes because they are
the only bytes that are checked by the current unpickle code. Thus
loading post-PEP fold=1 pickles in a pre-PEP Python will result in
an exception rather than an instance with out of range components.

No new implementations of datetime.tzinfo abstract class are
proposed in this PEP. The existing (fixed offset) timezones do
not introduce ambiguous local times and their utcoffset()
implementation will return the same constant value as they do now
regardless of the value of fold.

The basic implementation of fromutc() in the abstract
datetime.tzinfo class will not change. It is currently not used
anywhere in the stdlib because the only included tzinfo
implementation (the datetime.timezone class implementing fixed
offset timezones) overrides fromutc(). Keeping the default
implementation unchanged has the benefit that pre-PEP 3rd party
implementations that inherit the default fromutc() are not
accidentally affected.

New subclasses should override the base-class fromutc() method and
implement it so that in all cases where two different UTC times u0 and
u1 (u0 <u1) correspond to the same local time t,
fromutc(u0) will return an instance with fold=0 and
fromutc(u1) will return an instance with fold=1. In all
other cases the returned instance should have fold=0.

The utcoffset(), tzname() and dst() methods should use the
value of the fold attribute to determine whether an otherwise
ambiguous time t corresponds to the time before or after the
transition. By definition, utcoffset() is greater before and
smaller after any transition that creates a fold. The values returned
by tzname() and dst() may or may not depend on the value of
the fold attribute depending on the kind of the transition.

The sketch above illustrates the relationship between the UTC and
local time around a fall-back transition. The zig-zag line is a graph
of the function implemented by fromutc(). Two intervals on the
UTC axis adjacent to the transition point and having the size of the
time shift at the transition are mapped to the same interval on the
local axis. New implementations of fromutc() method should set
the fold attribute to 1 when self is in the region marked in
yellow on the UTC axis. (All intervals should be treated as closed on
the left and open on the right.)

If the utcoffset(), tzname() or dst() method is called on a
local time that falls in a gap, the rules in effect before the
transition should be used if fold=0. Otherwise, the rules in
effect after the transition should be used.

The sketch above illustrates the relationship between the UTC and
local time around a spring-forward transition. At the transition, the
local clock is advanced skipping the times in the gap. For the
purposes of determining the values of utcoffset(), tzname()
and dst(), the line before the transition is extended forward to
find the UTC time corresponding to the time in the gap with fold=0
and for instances with fold=1, the line after the transition is
extended back.

On ambiguous/missing times utcoffset() should return values
according to the following table:

fold=0

fold=1

Fold

oldoff

newoff = oldoff - delta

Gap

oldoff

newoff = oldoff + delta

where oldoff (newoff) is the UTC offset before (after) the
transition and delta is the absolute size of the fold or the gap.

Note that the interpretation of the fold attribute is consistent in
the fold and gap cases. In both cases, fold=0 (fold=1) means
use fromutc() line before (after) the transition to find the UTC
time. Only in the "Fold" case, the UTC times u0 and u1 are
"real" solutions for the equation fromutc(u) == t, while in the
"Gap" case they are "imaginary" solutions.

The value of the fold attribute will be ignored in all operations
with naive datetime instances. As a consequence, naive
datetime.datetime or datetime.time instances that differ only
by the value of fold will compare as equal. Applications that
need to differentiate between such instances should check the value of
fold explicitly or convert those instances to a timezone that does
not have ambiguous times (such as UTC).

The value of fold will also be ignored whenever a timedelta is
added to or subtracted from a datetime instance which may be either
aware or naive. The result of addition (subtraction) of a timedelta
to (from) a datetime will always have fold set to 0 even if the
original datetime instance had fold=1.

No changes are proposed to the way the difference t - s is
computed for datetime instances t and s. If both instances
are naive or t.tzinfo is the same instance as s.tzinfo
(t.tzinfo is s.tzinfo evaluates to True) then t - s is a
timedelta d such that s + d == t. As explained in the
previous paragraph, timedelta addition ignores both fold and
tzinfo attributes and so does intra-zone or naive datetime
subtraction.

Naive and intra-zone comparisons will ignore the value of fold and
return the same results as they do now. (This is the only way to
preserve backward compatibility. If you need an aware intra-zone
comparison that uses the fold, convert both sides to UTC first.)

The inter-zone subtraction will be defined as it is now: t - s is
computed as (t - t.utcoffset()) - (s -
s.utcoffset()).replace(tzinfo=t.tzinfo), but the result will
depend on the values of t.fold and s.fold when either
t.tzinfo or s.tzinfo is post-PEP. [5]

Note that the new rules may result in a paradoxical situation
when s == t but s - u != t - u. Such paradoxes are
not really new and are inherent in the overloading of the minus
operator differently for intra- and inter-zone operations. For
example, one can easily construct datetime instances t and s
with some variable offset tzinfo and a datetime u with
tzinfo=timezone.utc such that (t - u) - (s - u) != t - s.
The explanation for this paradox is that the minuses inside the
parentheses and the two other minuses are really three different
operations: inter-zone datetime subtraction, timedelta subtraction,
and intra-zone datetime subtraction, which each have the mathematical
properties of subtraction separately, but not when combined in a
single expression.

The aware datetime comparison operators will work the same as they do
now, with results indirectly affected by the value of fold whenever
the utcoffset() value of one of the operands depends on it, with one
exception. Whenever one or both of the operands in inter-zone comparison is
such that its utcoffset() depends on the value of its fold
fold attribute, the result is False. [6]

This exception is designed to preserve the hash and equivalence
invariants in the face of paradoxes of inter-zone arithmetic.

Formally, t == s when t.tzinfo is s.tzinfo evaluates to
False can be defined as follows. Let toutc(t, fold) be a
function that takes an aware datetime instance t and returns a
naive instance representing the same time in UTC assuming a given
value of fold:

This proposal will have little effect on the programs that do not read
the fold flag explicitly or use tzinfo implementations that do.
The only visible change for such programs will be that conversions to
and from POSIX timestamps will now round-trip correctly (up to
floating point rounding). Programs that implemented a work-around to
the old incorrect behavior may need to be modified.

Pickles produced by older programs will remain fully forward
compatible. Only datetime/time instances with fold=1 pickled
in the new versions will become unreadable by the older Python
versions. Pickles of instances with fold=0 (which is the
default) will remain unchanged.

While the tm_isdst field of the time.struct_time object can be
used to disambiguate local times in the fold, the semantics of such
disambiguation are completely different from the proposal in this PEP.

The main problem with the tm_isdst field is that it is impossible
to know what value is appropriate for tm_isdst without knowing the
details about the time zone that are only available to the tzinfo
implementation. Thus while tm_isdst is useful in the output of
methods such as time.localtime, it is cumbersome as an input of
methods such as time.mktime.

If the programmer misspecified a non-negative value of tm_isdst to
time.mktime, the result will be time that is 1 hour off and since
there is rarely a way to know anything about DST before a call to
time.mktime is made, the only sane choice is usually
tm_isdst=-1.

Unlike tm_isdst, the proposed fold attribute has no effect on
the interpretation of the datetime instance unless without that
attribute two (or no) interpretations are possible.

Since it would be very confusing to have something called isdst
that does not have the same semantics as tm_isdst, we need a
different name. Moreover, the datetime.datetime class already has
a method called dst() and if we called fold "isdst", we would
necessarily have situations when "isdst" is zero but dst() is not
or the other way around.

Suggested by Guido van Rossum and favored by one (but initially
disfavored by another) author. A consensus was reached after the
allowed values for the attribute were changed from False/True to 0/1.
The noun "fold" has correct connotations and easy mnemonic rules, but
at the same time does not invite unbased assumptions.

This was a working name of the attribute chosen initially because the
obvious alternative ("second") conflicts with the existing attribute.
It was rejected mostly on the grounds that it would make True a
default value.

The following alternative names have also been considered:

later

A close contender to "fold". One author dislikes it because
it is confusable with equally fitting "latter," but in the age
of auto-completion everywhere this is a small consideration. A
stronger objection may be that in the case of missing time, we
will have later=True instance converted to an earlier time by
.astimezone(timezone.utc) that that with later=False.
Yet again, this can be interpreted as a desirable indication that
the original time is invalid.

(Local Time Disambiguation Flag) - short and no-one will attempt
to guess what it means without reading the docs. (This abbreviation
was used in PEP discussions with the meaning ltdf=False is the
earlier by those who didn't want to endorse any of the alternatives.)

It has been suggested that backward compatibility can be improved if
the default value of the fold flag was None which would
signal that pre-PEP behavior is requested. Based on the analysis
below, we believe that the proposed changes with the fold=0
default are sufficiently backward compatible.

This PEP provides only three ways for a program to discover that two
otherwise identical datetime instances have different values of
fold: (1) an explicit check of the fold attribute; (2) if
the instances are naive - conversion to another timezone using the
astimezone() method; and (3) conversion to float using the
timestamp() method.

Since fold is a new attribute, the first option is not available
to the existing programs. Note that option (2) only works for naive
datetimes that happen to be in a fold or a gap in the system time
zone. In all other cases, the value of fold will be ignored in
the conversion unless the instances use a fold-aware tzinfo
which would not be available in a pre-PEP program. Similarly, the
astimezone() called on a naive instance will not be available in
such program because astimezone() does not currently work with
naive datetimes.

This leaves us with only one situation where an existing program can
start producing different results after the implementation of this PEP:
when a datetime.timestamp() method is called on a naive datetime
instance that happen to be in the fold or the gap. In the current
implementation, the result is undefined. Depending on the system
mktime implementation, the programs can see different results or
errors in those cases. With this PEP in place, the value of timestamp
will be well-defined in those cases but will depend on the value of
the fold flag. We consider the change in
datetime.timestamp() method behavior a bug fix enabled by this
PEP. The old behavior can still be emulated by the users who depend
on it by writing time.mktime(dt.timetuple()) + 1e-6*dt.microsecond
instead of dt.timestamp().

The time.mktime interface allows three values for the tm_isdst
flag: -1, 0, and 1. As we explained above, -1 (asking mktime to
determine whether DST is in effect for the given time from the rest of
the fields) is the only choice that is useful in practice.

With the fold flag, however, datetime.timestamp() will return
the same value as mktime with tm_isdst=-1 in 99.98% of the
time for most time zones with DST transitions. Moreover,
tm_isdst=-1-like behavior is specified regardless of the value
of fold.

It is only in the 0.02% cases (2 hours per year) that the
datetime.timestamp() and mktime with tm_isdst=-1 may
disagree. However, even in this case, most of the mktime
implementations will return the fold=0 or the fold=1
value even though relevant standards allow mktime to return -1 and
set an error code in those cases.

In other words, tm_isdst=-1 behavior is not missing from this PEP.
To the contrary, it is the only behavior provided in two different
well-defined flavors. The behavior that is missing is when a given
local hour is interpreted as a different local hour because of the
misspecified tm_isdst.

For example, in the DST-observing time zones in the Northern
hemisphere (where DST is in effect in June) one can get

Thus one of the suggested uses for fold=-1 -- to match the legacy
behavior -- is not needed. Either choice of fold will match the
old behavior except in the few cases where the old behavior was
undefined.

Another suggestion was to use fold=-1 or fold=None to
indicate that the program truly has no means to deal with the folds
and gaps and dt.utcoffset() should raise an error whenever dt
represents an ambiguous or missing local time.

The main problem with this proposal, is that dt.utcoffset() is
used internally in situations where raising an error is not an option:
for example, in dictionary lookups or list/set membership checks. So
strict gap/fold checking behavior would need to be controlled by a
separate flag, say dt.utcoffset(raise_on_gap=True,
raise_on_fold=False). However, this functionality can be easily
implemented in user code:

Moreover, raising an error in the problem cases is only one of many
possible solutions. An interactive program can ask the user for
additional input, while a server process may log a warning and take an
appropriate default action. We cannot possibly provide functions for
all possible user requirements, but this PEP provides the means to
implement any desired behavior in a few lines of code.

This image is a work of a U.S. military or Department of Defense
employee, taken or made as part of that person's official duties. As a
work of the U.S. federal government, the image is in the public
domain.